High efficiency low pressure plasma source has been widely used in neutron generator, downstream plasma cleaner, molecular beam epitaxy (MBE), ion and electron beam production. In many applications, the remote plasma sources are attached to a downstream vacuum chamber. The pressure inside the downstream vacuum chamber should be kept as low as possible. For example, the downstream vacuum chamber pressure for plasma ion source assembly should be kept below 10−4 torr to avoid the risk of arcing. In downstream plasma cleaning or deposition applications, reactive radicals generated in the plasma source diffuse into the downstream sample chamber to carry out etching or deposition process on the sample surface. If the pressure in the downstream sample chamber is too high, radicals may recombine and loose reactivity in the downstream sample chamber. An obvious and well-known solution is to use a differential pumping aperture between the plasma source and the downstream sample chamber. Plasma sources equipped with differential pumping aperture or flow restrictor have been disclosed in many prior arts, such as U.S. Pat. Nos. 3,961,103, 5,788,778 and 6,749,717. However, flow restrictor can also limit the transportation of the radical species into the processing chamber, as pointed out by U.S. Pat. No. 4,088,926.
Plasma ion sources also use an aperture plate for differential pumping and beam shaping purpose, such as the design published in Review of Scientific Instruments V74, 2288(2003). To obtain a broader ion or electron beam, a beam shaping aperture with wider openings will be required. However, wider opening on the aperture plate will greatly reduce the pressure difference between the plasma source and the downstream vacuum chamber. High pressure inside the downstream vacuum chamber can significantly increase the risk of arcing between high voltage electrodes. The plasma source design disclosed here improved the performance of traditional ICP plasma ion sources at low pressure conditions. It increased the plasma strength by 2 to 3 times compared with traditional ICP plasma sources under similar conditions. It can ignite and sustain a plasma with only 0.1 mTorr pressure inside the plasma source if air is used as the process gas. The improved performance is achieved by adding an extra booster tube to the traditional ICP plasma chamber.
The booster tube in this application has some similar properties as the traditional hollow cathode or hollow anode electrode. But there is no cathode or anode electrode in inductively coupled plasma source or microwave plasma source. The booster chamber design disclosed in this invention is an attachment to the discharge chamber of fully functionally plasma sources. Yet it greatly improves the plasma density at low pressure through a positive feedback mechanism. Traditional hollow cathode or hollow anode is usually designed to be part of an electrode that usually has a bias voltage, such as U.S. Pat. Nos. 4,871,918, 4,954,751 and 6,452,315. In traditional hollow cathode or hollow anode plasma source, high rf or DC voltages are applied to the metal electrodes to initiate the plasma ionization process and to couple discharge energy into the plasma source. Once the plasma is ignited, ions will be accelerated to relatively high energy by the voltage applied on the metal electrodes. Energetic ions may cause sputtering damage to the hollow cathode or hallow anode electrodes. The booster chamber design disclosed in this invention greatly reduces the risk of ion sputter damage because there is no high voltage applied to the booster chamber. It not only improves the discharge efficiency, reduces the operation pressure, but also increases the reliability and lifetime of the plasma source.
Downstream plasma processing relies on the radicals generated inside the plasma source to diffuse into the downstream processing chamber to carry out etching or deposition processes. Usually the downstream processing chamber is much larger than the plasma chamber. Overall pumping speed of the downstream processing chamber could be more than 10 times higher than the pumping speed from the plasma source chamber because of the differential pumping aperture between the plasma source and the downstream sample chamber. As a result, the density of the radicals in the downstream sample chamber is significantly lower than the density of the radicals in the plasma source. If high speed etching and deposition is required in some applications, samples to be processed should be immersed in the plasma. Since there is no rf antenna inside the booster chamber, small samples can be placed inside the booster chamber without changing plasma discharge efficiency too much. Booster chamber can be designed into an assembly that can be easily detached from the main plasma source. Or it can be designed to have an entry port for user to load small samples. Even though there is no rf antenna or electrodes inside the booster chamber, the plasma density is similar to other part of the plasma source. Compared with downstream mode plasma processing, the etching and deposition speed inside the booster chamber can be increased by more than one order of magnitude. Even though booster chamber can only process relative small samples, it provides an alternative way to carry out high speed immersion mode plasma processing on a remote plasma source.
Traditionally the strength of the plasma is gauged by the ion and electron density derived from the I-V characteristics obtained in Langmuir probe measurement. To operate a Langmuir probe, high voltage power supply and complicated control electronics are required. The cost of implementing Langmuir probe into control electronics is usually quite high. Plasma emission spectrum has been widely used in plasma diagnostics. For example, researchers have developed methods to monitor the progress of the plasma etching process by measuring plasma emission intensity from certain reaction byproducts. Usually one or plural optical bandpass filters are used to select plasma emission from certain gas species. Composition of the gaseous species inside the plasma processing chamber will change at different stages of the plasma etching process. By measuring the light emission from specific gaseous species, users can monitor the change of the concentration level for such gaseous species inside the plasma processing chamber. This kind of plasma emission diagnostic has been widely used in traditional plasma processing equipment for end-point detection, such as U.S. Pat. No. 5,045,149. Plasma emission spectrum has also been used as a feedback mechanism for tuning rf matching in plasma reactors (U.S. Pat. No. 8,144,329). A laser welding method disclosed in U.S. Pat. No. US 2007/0289955 used the light emission strength of the plasma or plume to control the laser output in order to prevent occurrence of welding defects.
A method to gauge the strength of a remote plasma source by measuring the total plasma emission intensity or intensity within certain wavelength range has been disclosed in this invention. Unlike the end-point detection applications, the plasma emission intensity data in this method is not intended to monitor the change of the gas composition in the plasma source or in the processing chamber. Instead, it is used to gauge the strength of the remote plasma source. Usually, the gas composition in the remote plasma source doesn't change unless the gas delivery system changes the input gas composition. U.S. Pat. No. 2007/0289955 and U.S. Pat. No. 5,304,774 have disclosed methods to monitor light emission from plasma during laser welding to improve welding process. This is the first time the intensity of the plasma emission is used to gauge the strength of the plasma in remote plasma sources. Compared with the traditional Langmuir probe method, method disclosed in this invention is much easier to implement. Plasma emission intensity can be measured by photodiode, photoresistor or phototransistor. Most of these light sensors can be easily integrated into the control electronics at a very low cost.